The present invention relates generally to a shield for a sampling or monitoring unit or probe which is inserted into a gas flow to measure, monitor or sample one or more gases. More particuarly, the present invention relates to a shield for a probe of an oxygen, sulfur dioxide and nitrous oxide monitoring or sampling unit, the respective probe being surrounded by a porous filter and extending into a gas flow typically within a stack, transition piece or other gas conduit or the like to monitor or sample the oxygen, sulfur dioxide and nitrous oxide contents of the gases passing therethrough.
Combustion or process gases are monitored for environmental purposes to determine emissions prior to release to the atmosphere, to determine combustion efficiency, and for other purposes. Probes used to sample such gases are typically inserted into a stack or other gas conduit such as a transition piece, and must operate under varied conditions of heat and exposure to gases, water, steam and particulate matter.
For example, in sampling the combustion products of coal, a probe can be exposed to temperatures typically in the range of about 60.degree. F. to about 200.degree. F. and higher, and can be exposed to gases such as sulfur dioxide, nitrous oxide, carbon dioxide and carbon monoxide, to steam and water, and to particulate matter such as soot and ash. The particulate matter which may reach the probe is typically extremely fine, having escaped removal by scrubbing systems and electrostatic precipitators located upstream (relative to the gas flow) of the probe. A ceramic or other porous filter is provided to enclose the probe, thereby filtering any particulate matter which has reached the filter and preventing such particulate matter from reaching the probe itself, while permitting the gases to pass through the filter and reach the probe. The amount of water that the filter is exposed to can be considerable, particularly on cool days when the stack or conduit carrying the gases is at the dew point, thereby causing condensation of water vapor in the stack or conduit. During the use of such a probe, there is a tendency for water to accumulate on the exterior surface of the filter. The water causes the particulate matter which, as mentioned, is extremely fine, to become moistened and to adhere to and cake or become encrusted on the filter. In a very short time, the filter becomes entirely clogged or "blinded" by the encrusted particulate matter.
In one prior art oxygen probe having a ceramic filter, the ceramic filter was exposed directly to the gases, water and particulate matter except for a portion of the ceramic filter which faced upstream of the gas flow. That portion was shielded by a V-shaped member. The apex of the V faced upstream of the gas flow and acted to blunt the flow of gases and particulate matter around the ceramic filter. However, a substantial portion of the ceramic filter was directly exposed, and water and particulate material tended to accumulate on the filter, as mentioned above. As a result, the particulate matter became moistened and caked on the ceramic filter. In a short time, not unusually after only a few days of use, the ceramic filter become encrusted and clogged.